The invention relates to electromagnetic energy supporting structures and more particularly to microwave heating apparatus having energy seals to prevent the leakage of such energy from the apparatus. In the microwave oven art the escape of high frequency energy from an oven cavity is desirably controlled in order to comply with standards established by State and Federal regulatory agencies and bodies, such as the Department of Health, Education and Welfare, Federal Communication Commission, and The United States of America Standards Institute. Conventionally, such apparatus operates at assigned frequencies of either 915 or 2,450 MHz and the term "microwave" as used in this description of the invention is intended to refer to that portion of the electromagnetic energy spectrum having wavelengths in the order of 1 meter to one millimeter and frequencies in excess of 300 MHz.
In microwave ovens the energy fed to the oven interior preferably resonates in a plurality of modes achieved by suitable adjustment of the oven dimensions. Such resonant modes are loaded by absorption of heat by the article being heated, and such absorption varies with the absorbing characteristics of the article as well as its size and shape. To assure uniformity of heating it has generally been desirable to cyclically vary the mode pattern with respect to the article by, for example, a mechanical mode stirrer, by movement of the article within the oven, by varying the frequency of the energy fed to the enclosure, and/or combinations of all the foregoing. The multiplicity of modes which vary with the loading within the oven and with the cyclical variation of the modes therein can result in excitation of modes within a door seal which have propagating components along the seal and can produce undesirably large amounts of energy leakage through the door seal.
In the prior art numerous energy seals have evolved including those providing metal-to-metal contacting surfaces or interdigital structures, such as, for example, the embodiments referred to in U.S. Pat. Nos. 2,956,143 issued to L.H. Shall on Oct. 11, 1960, 2,958,754 issued to D.E. Hahn on Nov. 1, 1960, as well as 3,448,232 issued to J.H. Kluck on June 3, 1969. Among the disadvantages with such structures is the mechanical variations which over a period of time develop gaps with substantial energy leakage. It is also inherent in such structures and, in particular, the last named structure that propagation of energy in all modes and in all directions along the energy seal must be suppressed by substantially complete metal-to-metal contact in order for the seal to be effective. Additionally, with the establishment of gaps over periods of time between the contacting metal surfaces high frequency energy arcing may occur during operation and/or upon opening of the door.
Other prior art energy seals include electrical choke arrangements together with dielectric bodies to define paths of least resistance for energy leaking along the peripheral gap defined between the door and access opening walls of an oven. Several examples of choke energy seals are illustrated in U.S. Pat. Nos. 3,182,169 issued to Richard Ironfield on May 4, 1965, and 3,584,177 issued to Arnold M. Bucksbaum on June 8, 1971. The choke type energy seals are intended to handle single energy propagating modes and have, for example, a dimension of one-quarter of a wavelength of the operating frequency of a TEM-mode along a first path with a total excursion of one-half a wavelength from a short circuit defined by a terminating wall which is reflected back to the point of origin of the escaping energy through the peripheral gap.
In previous embodiments such choke arrangements in the course of time may change their electrical characteristics due to mechanical wear and buildup of food particles which alters the door dimensions and hinders the effectiveness of the choke in preventing leakage. As a result, heating apparatus which initially meets the low radiation standard levels after installation may significantly drift from such levels. It has further been noted that with the excitation of plural modes within the oven cavity the region defined by the peripheral gap around the access opening becomes an efficient propagating structure for, particularly, modes propagating in a y-direction peripherally around the gap, as well as such energy modes intended to be directed across the energy seal, hereinafter referred to as the x-direction, to encounter the choke arrangement. A wide range of higher-order modes can be initiated which propagate in the x-direction having wavelengths greatly different than the operating frequency wavelength because these modes are associated with cutoff resonance of y-propagating modes at frequencies close to the operating frequency.
In attempting to understand the electrical characteristics of the undesired modes propagating peripherally along the energy seal associated with the x-directed modes a study of the longitudinal and transverse current flows has indicated that energy seals, particularly those with choke arrangements, exhibit large quasi-periodic variations of leakage superimposed upon or even counteracting the expected TEM-mode suppression characteristics of the choke. Energy absorbing bodies, such as gaskets, are, therefore, coupled just beyond or outside the mode supporting energy seals in order to further dissipate energy leakage and assure maintenance of the standards. If the dielectric bodies within the choke or the energy absorbing bodies do not have stable dimensions and locations during the operation of the oven apparatus or if the door location and centering varies then excessive energy leakage and gasket heating will occur in an uncontrollable manner. A need arises, therefore, for an improved high frequency energy apparatus having an energy seal which will provide for controlled energy mode propagation in one direction by inhibiting the initiation of modes which can propagate in a different direction.